The present invention relates to a device for a camera for displaying directly in the viewfinder of the camera an indication that the photographing lens is focused on the object in response to a contrast signal extracted from the image of an object to be photographed.
A variety of different focus detecting devices have been proposed in the art. These include a so-called "automatic focusing camera" in which the photographing lens is automatically positioned at a point where it is focused on the desired object, hereinafter referred to as "a focus position" when applicable. However, automatic focusing devices for a single-lens reflex camera are not currently in widespread commercial use. One of the important reasons for this is that, as a single-lens reflex camera is essentially a high grade camera, its focus detection accuracy must be extremely high. In addition, due to the reasons enumerated below, it is rather difficult to manufacture automatic focusing single-lens reflex cameras. That is, as most lenses used with a single-lens reflex camera are relatively heavy, the lens driving device must necessarily be large. In order to stop the photographing lens at the desired focus position, it is necessary to determine whether the lens is currently positioned in front of the focus position or behind it. The circuit required for performing these functions is necessarily intricate.
A focus detecting technique has been proposed which utilizes the fact that the optical power spectrum of the image of an object to be photographed becomes a maximum at the point of focus. For instance, in one implementation of such a technique, the image of an object is mechanically or electrically scanned with a photoelectric transducing element. More specifically, a time-series scanning output signal corresponding to the brightness distribution of the image is obtained by scanning the image. The signal thus obtained is differentiated thereby to extract high spatial frequency components which contain the desired contrast information. The differentiated signal is converted into an absolute value waveform. Then, the peak value of the absolute value waveform for each scanning period is detected and employed as a focusing signal.
Recently, a technique using a self-scanning type photoelectric transducing element as the scanning means has been disclosed in the art. The self-scanning type photoelectric transducing element is made up of a plurality of microphotoelectric transducing elements and a scanning circuit. Such an element is available on the market as a "MOS-FET type" device or a "CCD type" device depending on the type of scanning circuit employed. The use of such a photoelectric transducing element makes the scanning section compact so that it can be readily incorporated in a single-lens reflex camera or the like.
In order to stop the photographing lens at the focus position where the peak value becomes a maximum, it is necessary to provide some means for determining that the contrast signal is at a maximum. For this purpose, heretofore either two groups of photoelectric transducing elements were disposed respectively in front of and behind the focal plane on the optical axis or a memory circuit was used to compare signals representative of two different extension positions of the photographing lens.
The contrast signal can be produced without using differentiation. For instance, the contrast signal can be produced by a technique in which a number of microphotoelectric transducing elements are arranged in a plane in which the image of an object is formed and the difference of the photoelectric outputs of a pair of adjacent elements is determined.